The murine legless transgene insertional mutation results in a remarkable phenotype resulting from a large deletion at the distal end of chromosome 12. Mutants exhibit a number of developmental abnormalities, including truncated limbs, brain malformations, and randomized left-right axis. We cloned a molecular motor dynein gene, Lrd, from the legless deleted region. This gene is expressed only in the node at E7.5, and targeted mutation confirmed its importance in left-right axis development. Specific aim one will test molecular models of Lrd left-right function. The nodal flow model predicts that LR]) contributes to movement of nodal cilia, thereby creating an extraembryonic morphogen gradient. Other models predict cytoplasmic function for LRD in the node. We propose to target the Lrd gene to make a LRD-GFP fusion protein, allowing precise intracellular localization. We also propose to directly examine the arrangement of microtubules in the cells of the node to search for aligned arrays of microtubules, which are predicted by certain models. We also cloned the Sp4 transcription factor gene from the legless deleted region. It was a good candidate for involvement in the legless limb phenotype because of its homology to a family of genes in Drosophila that includes Distalless, which gives truncated appendages when mutated. Nevertheless, targeted mutation of Sp4 did not give limb malformations. Recently, however, we have discovered the presence of a second Sp gene, Sp7, closely linked to Sp4. In Drosophila the two orthologues of these Sp genes are tightly linked, functionally redundant, and involved in limb development. In specific aim two we propose to characterize the murine Sp7 gene by defining its genomic organization and developmental expression profile in wild types and legless mutants. Initial results have shown that in wild type embryos Sp7 is expressed in the forming AER, which plays a critical role in limb outgrowth. This expression is absent in legless mutants. We therefore hypothesize that the mutation of Sp7, perhaps in combination with the deletion of a functionally redundant Sp4, causes the limb defects of the legless mutant. In specific aim 3, we propose a series of genetic tests of this hypothesis. The results will provide deeper insight into the genetic program of limb development and might also find function for these genes in the formation of the brain and other organs.